In this specification, the term optical is intended to refer to that part of the electromagnetic spectrum which is generally known as the visible region together with those parts of the infra-red and ultra-violet regions at each end of the visible region which are capable of being transmitted by dielectric optical waveguides such as optical fibres.
Commonly, the amplitude of an optical carrier signal is modulated in accordance with information to be transmitted. It is preferable, however, in certain applications for example coherent systems for this information to be transmitted as a phase modulation of a carrier signal.
In accordance with one aspect of the the present invention, an optical communication system comprises a transmission assembly including a conversion device to which an optical signal whose amplitude is modulated in accordance with information is fed, the conversion device being responsive to the power of the amplitude modulated optical signal to produce an optical output signal whose phase is modulated in correspondence with the amplitude moduation of the incoming signal; and a receiving assembly for detecting the phase modulation to regenerate the information.
In the simplest example, the conversion device superimposes a phase modulation on the incoming amplitude modulated signal. In this case, the signal produced will be both and amplitude and phase modulated. Conveniently, therefore, the transmission assembly further comprises separation means for receiving the output signal from the conversion device and for producing a solely phase modulated signal. An example of a suitable separation means is provided by a saturated laser amplifier.
The amplitude modulated optical carrier signal may be generated in any conventional manner before being fed to the transmission assembly.
In accordance with a second aspect of the present invention, a transmission assembly comprises a conversion device for receiving an amplitude modulated optical signal and for producing an optical output signal whose phase is modulated in correspondence with the amplitude moduation of the incoming signal; an auxiliary optical signal source; and feed means for feeding signals from the auxiliary source to the conversion device.
In one arrangement, the signal generated by the auxiliary source comprises a carrier signal, the feed means comprising an optical coupling device having an input port connected to the auxiliary source and an output port connected with the conversion device.
In this arrangement the auxiliary signal is phase modulated by the amplitude modulated signal.
In another arrangement, the feed means comprises an optical coupling device having a pair of input ports one of which is connected to the auxiliary source and the other of which receives a modulated optical signal, and an output port connected to the conversion device whereby the signal from the auxiliary source is combined in use with the modulated optical signal to produce an amplitude modulated signal.
In this arrangement, the transmission assembly can act as a repeater to amplify the incoming signal. Furthermore, it allows a phase modulated signal to be amplified.
Preferably, the conversion device comprises an optical device having a refractive index which varies in accordance with the power of the amplitude modulated optical signal. An example of such an optical device is provided by a semiconductor laser amplifier.